Studies will be directed toward the elucidation of humoral immune mechanisms that may be operative (1) in experimental and human syphilis and (2) following vaccination with gamma-irradiated Treponema pallidum, utilizing the methods of passive protection and neutralization. Studies will also be directed toward the elucidation of cell-mediated mechanisms that may be operative utilizing a method for determining the direct cytotoxic effect of lymphocytes upon T. pallidum.